Belt structures, particularly structures of composite belts for carrying accessories, are confronted with conventional problems of the variation of the size of the users. Customarily, two solutions have been adopted: either there are provided different belt structures for different sizes, with the resulting drawback of multiplication of stock and inventory and the lack of versatility of the belt structure for different users of different sizes, or the adaptation to different sizes by means of the securement of the free end of the free portion, with the drawback in use particularly in difficult environments and/or with the use of gloves, as in underwater diving, of manipulation and maintenance of the excessive length of the free portion.